1. Field of the Invention
The present invention relates to an optical disc apparatus that writes information onto or reads information from an optical disc, and more particularly to a technology for exercising ideal focus control over an optical disc having a plurality of recording layers.
2. Description of the Related Art
For example, a focus error signal derived from the use of a conventional astigmatism detection (CAD) system or a differential astigmatism detection (DAD) system is conventionally used for focus control over an optical pickup objective lens of an optical disc apparatus. The CAD system is used to detect zero-order light, which is included in reflected laser light that has passed through the objective lens, perform an addition/subtraction process on the zero-order light, and form the focus error signal. The DAD system is used to detect zero-order light and ±first-order light, which are included in the reflected laser light that has passed through the objective lens, perform an addition/subtraction process on the zero-order light and ±first-order light, and form the focus error signal.
The CAD system has a simple configuration, but is susceptible to disturbance. On the other hand, the DAD system can provide accurate focus control because it can suppress the leakage of reflected laser light from an adjacent track when a laser beam spot, which is formed on an optical disc surface due to light condensation by the objective lens, crosses a recording track while a tracking servo is OFF. The DAD system is often used because it, for instance, assures stability during a seek operation and reduces the power consumption.
A technology for providing improved focus control is disclosed, for instance, by JP-A-2000-82226. For a DVD-RAM disc, this technology uses the focus error signal derived from the DAD system to exercise focus control over the objective lens. For a DVD-ROM disc, however, this technology uses the focus error signal derived from the CAD system to exercise focus control over the objective lens. Another technology for providing improved focus control is disclosed, for instance, by JP-A-2005-174421. This technology obtains the focus error signal through the so-called DAD system and a tracking error signal through the use of a differential push-pull method. More specifically, to considerably reduce the influence of disturbance that affects the focus error signal and an offset that affects the tracking error signal, this technology provides a selector switch for a circuit that performs an addition/subtraction process on signals fed from optical detectors, and operates the selector switch so that the divided regions of an output source for signals of two optical detectors, which are to be added, belong to the same optical detector or belong to the regions of the two optical detectors.